Veteran QBs past their prime (main team in parentheses) whom the Rams picked up over the years that mostly didn't pan out:
John Hadl (Chargers), 1973: Got the MVP that year and was good until the playoffs. Traded in 1974.
Joe Namath (Jets), 1977: Led Rams to a 2-2 record before warming the bench. Retired after season.
Dan Pastorini (Oilers), 1981: Was crappy on a losing Rams team. Retired after season.
Bert Jones (Colts), 1982: Played decent until a career ending neck injury. Retired after season.
Dieter Brock (Hamiton, Canada), 1985: We know this story too well.
Chris Miller (Falcons), 1994-95: started 23 games but retired re: concussion issues.
Chris Chandler (Falcons), 1994-95, 04: started 8 games in backup role. Injuries: yes. (best season in 1998)
Mark Rypien (Redskins), 1995,97: started 3 games in backup role.
Gus Frerotte (Redskins), 2006-07: started 2 games in backup role.
Nick Foles (Eagles), 2015: started 11 games.
Some had prev won championships, some led teams to playoffs, some were all-pro, a few just had good years with other teams before Rams landed them.